Prevention of marine encrustation on bronze propellers

ABSTRACT

From the time that they are immersed into a marine environment, bronze propellers are prone to attack by marine organisms, such as barnacles, coral and algae, which attach themselves to the bronze metallic surface, creating lumps on the propeller, which adversely affect its balance and cause impedance and vibration of the propeller and its boat in the water. Anti-fouling paints are either too toxic for the marine environment or lack smoothness on the surface. These problems have been overcome by polishing the propeller to prepare it for electroplating, cleansing to remove dirt and grease, electroplating with copper, followed by spraying with a standard solution (5%) of sodium hypochlorite and sodium chloride and allowing sufficient time for a reaction of the hypochlorite solution with the copper to form a firmly adhering conversion coating of basic cupric chloride. The coating is blue-green in color.

This invention relates to a process for the prevention of marineencrustation on components used in a marine environment and inparticular although not exclusively bronze or copper components, such asbronze propellers. Bronze includes tin bronze, aluminium bronze,silicon-aluminium bronze, nickel-aluminium bronze and manganese bronze.

A problem with components that are submerged in a marine environment forextended periods is that marine life encrustation can develop on thesurface of the component. This is particularly a problem withpropellers. In operation, water travels over the propeller blades athigh velocity, it is essential for the efficiency of the propeller, thatthe surface be perfectly smooth, even and true. From the time that theyare immersed into a marine environment, bronze propellers are prone toattack by marine organisms, such as barnacles, coral and algae, whichattach themselves to the bronze metallic surface, creating lumps on thepropeller, which adversely affect its balance and cause impedance andvibration of the propeller and its boat in the water. Various remedieshave been tried including anti-fouling paints. One of these paintscontaining tributyl tin, was so toxic to other economic marine life,such as oysters, that it had to be discontinued. Its successor is soaggressive, that its application to the finely polished surface of thebronze leaves brush marks in the form of grooves, that adversely affectthe fine balance and vibration free performance of the propeller.Durability of the anti-fouling paint on the propeller can be as short as30 days in active marine environments.

Accordingly, the inventive process seeks to provide protection frommarine encrustation for an extended period of time.

In accordance with the present invention there is provided a process forimproving the resistance to marine encrustation of a component used in amarine environment, including the step of treating the component with ahypochlorite or a hypochlorite-containing material or a precursorthereof.

Preferably, the component is made from copper, contains copper or isprovided with a surface layer of copper or copper-containing material.More preferably, the process further includes forming a layer of copperor copper-containing material on the surface of the component beforetreatment by the hypochlorite material, particularly if the surface ofthe component is not copper or is not provided with a copper orcopper-containing surface.

Preferably, the hypochlorite material is a hypochlorite salt solution ora material capable of forming a hypochlorite salt solution.

Preferably, a cupric chloride coating is formed on the surface of thecomponent by treating the copper surface or copper surface layer of thecomponent with the hypochlorite salt solution.

Preferably, the component is metallic and the copper layer is formed onthe metallic surface by electroplating. More preferably, the copperlayer is formed to a minimum depth of 0.06 millimetres. Typically thecomponent or the surface of the component is or includes bronze beforebeing electroplated with copper.

Preferably, a process according to claim 6, wherein the bronze surfaceis cleansed before electroplating.

Preferably, the copper surface is exposed to the hypochlorite saltsolution for a period of time sufficient for the basic cupric chloridecoating to form.

Preferably, the hypochlorite salt solution is a standard sodiumhypochlorite solution containing 5% sodium hypochlorite and 5% sodiumchloride.

Preferably, the hypochlorite salt solution is applied, preferablysprayed onto the copper surface or copper layer.

Preferably, the surface of the component is polished before it iscleansed.

Preferably, the step of forming the basic cupric chloride coating ispreceded by the formation of cupric oxide.

Preferably, the component is a bronze propeller. More preferably, thebronze propeller is a newly cast bronze propeller that has been firstpolished to propeller production standard. Preferably, wherein thepolishing procedure incudes using a 60# grit size at 3500 sfm forroughing, followed by using a 180# grit size at 5500 sfm for finishingthe propeller using grease as a polishing aid.

Preferably, the surface cleansing includes one or more steps of alkalinecleaning by dipping or electrolytic means, vapour degreasing and solventcleaning.

Preferably, the copper layer is electroplated using an electroplatingbath which is alkaline. More preferably, the copper electroplating bathis an alkaline cyanide bath. Alternatively, the copper electroplatingbath is an alkaline pyrophoshate bath.

Alternatively, the copper layer is electroplated using an electroplatingbath which is acid. Preferably, the copper electroplating bath is anacid sulphate bath. Preferably, the copper electroplating bath is anacid fluoborate bath.

In accordance with another aspect of the present invention there isprovided a component having increased resistance to marine encrustationin a marine environment, wherein the surface of the component has beentreated with a hypochlorite or hypochlorite-containing material orprecursor thereof, after which the component is provided with a basiccupric chloride coating.

Preferably, the component is made from copper, contains copper or isprovided with a surface layer of copper or copper-containing material onwhich the basic cupric chloride coating is formed. More preferably, thecopper or copper-containing surface is a layer formed on a metallicsurface of the component. Still more preferably, the metallic surface isa bronze surface of the component: Typically, the component is a bronzepropeller.

Preferably, the layer is an electroplated layer.

Preferably, the basic cupric chloride coating is a cupric oxy-chloridecoating. More preferably, the basic cupric chloride coating is a cupricchloro-hypochlorite coating.

In order to provide a better understanding of the present invention, aembodiment will now be described in detail. The preferred embodiment isdescribed in relation to bronze propellers, however it will beappreciated that the invention is applicable to other components.

The process for the prevention of marine encrustation on bronzesurfaces, in particular those of propellers, is characterised by thefollowing steps:

(a) The cast propeller is first polished to the propeller productionstandard.

(b) The whole polished propeller is then cleansed, to remove all tracesof dirt and grease.

(c) The cleansed propeller is then electroplated with copper preferablyto a depth of about 0.002″ or 0.06 mm.

(d) The electroplated propeller is then placed in a suitable containerand sprayed with a standard solution of sodium hypochlorite.

Thereafter, only minimum maintenance is required, when the vessel isslipped periodically, thus providing a substantial reduction inmaintenance costs.

The first step in the preparation is polishing. Bronze propellers andassociated structure are typically sand cast and require polishing toremove scale. Reference top this procedure may be found in the article“Mechanical Finishing—Polishing and Buffing”. The recommended procedureis 60# grit size at 3500 sfm for roughing, followed by 180# grit size at5500 sfm for finishing using grease as a polishing aid.

The second step in the procedure consists of surface preparation orcleansing to remove all traces of dirt and grease and may consist of oneor more of alkaline cleaning by dipping or electrolytic means, vapourdegreasing and solvent cleaning in the article ²“Metal Cleaning—Sectionof Cleaning Process”. This surface preparation is also the subject ofstandard ASTM B281—“Preparation of Copper and Copper Base Alloys forElectroplating”.

The third step involves electroplating the cleansed propeller withcopper to a depth of, for example 0.002″ or 0.06 mm. Various salts ofcopper may be used, but the most common are those of the two alkaline(cyanide and pyrophosphate) baths and the two acid (sulphate andfluoborate) baths. These are variously described in ³“Copper Plating” byMattie F. McFadden and are the subject of two standards AMS 2418 andMII,-C-14550(Ord). As well as provided an appropriate surface forsubsequent processing, electroplating with copper enhances the surfaceby providing a substantial levelling effect ranging from 70 to 90 percent for a deposit of 0.005″ in thickness.

The fourth step in the process consists of placing the electroplatedpropeller in a suitable container and spraying the electroplated surfacewith sodium hypochlorite solution. This solution is prepared bychlorination of sodium hydroxide (caustic soda) solution

2NaOH+Cl₂→NaCl+NaClO+H₂O

or, alternatively, by electrolysis of a sodium chloride (common salt)solution and reacting the product of the anode (chlorine) with theproduct of the cathode (sodium hydroxide). Sodium hypochlorite isroutinely marketed as a 5% equimolecular solution of sodium chloride andsodium hypochlorite for the disinfection and sterilisation of suchplaces as dairies and milking sheds under various trade names such asEau de Javelle, Chlorox and Dazzle. The reactions with take place withthe copper surface are believed to be

 Cu+NaClO→CuO+NaCl  -(1)

(black)

A first coating of black cupric oxide is formed in reaction (1), whichcoating is then converted to a blue-green basic cupric chloride inreaction (2). The basic cupric chloride forms a firmly adherent coating,which resists the encroachment of marine organisms. A life expectancy offive years of effective protection against. marine growth has beenachieved, providing ultimate thrust to manufacturers' standards,together with precise balance and vibration-free performance.

It will be apparent to persons skilled in the relevant arts thatmodifications and variations can be made to the described inventionwithout departing from the basic inventive concepts, such as:

(i) the surface treated to form the basic cupric chloride coating mayalready be copper and therefore copper electroplating may not benecessary;

(ii) a component may not necessarily be bronze, so long as it iselectroplated with copper on which is formed the basic cupric chloridecoating;

(iii) a standard solution of sodium hypochlorite may be substituted withother hypochlorite salt solutions; and,

(iv) the hypochlorite salt solution may be applied in various other waysto the copper surface than spraying, such as painting or dipping.

All such modifications and variations as would be apparent to a skilledaddressee are intended to be included with the scope of the presentinvention, the nature of which is to be determined from the foregoingdescription and appended claims.

BIBLIOGRAPHY

^(1,2,3) Metals Handbook, 8th edit., Vol. 2-“Heat Treating, Cleaning andFinishing”. (ASM, Metals Park, Ohio 1964).

What is claimed is:
 1. A process for improving resistance to marineencrustation of a component used in a marine environment including thesteps of: providing a component adapted for use in a marine environment,said component having an outer surface substantially covered withcopper; applying a hypochlorite solution to said outer surface; andallowing sufficient time for a reaction of said hypochlorite solutionwith the copper to form a blue-green layer at the outer surface, whereinthe blue-green layer formed at the outer surface resists marineencrustation of said component in a marine environment.
 2. A process inaccordance with claim 1, wherein the hypochlorite solution is ahypochlorite salt solution or a material capable of forming ahypochlorite salt solution.
 3. A process according to claim 2, whereinthe hypochlorite salt solution is a sodium hypochlorite solutioncontaining 5% sodium hypochlorite and 5% sodium chloride.
 4. A processin accordance with claim 1, wherein the blue-green layer includes abasic cupric chloride.
 5. A process according to claim 1, wherein thestep of applying the hypochlorite material is performed by spraying thehypochlorite material onto the component.
 6. A process for improvingresistance to marine encrustation of a component used in a marineenvironment including the steps of: providing a component adapted foruse in a marine environment; applying a layer of copper or asubstantially copper-containing material to an outer surface of thecomponent; applying a hypochlorite solution to said outer surface; andallowing sufficient time for a reaction of said hypochlorite solutionwith the copper to form a blue-green layer at the outer surface, whereinthe blue-green layer formed at the outer surface resists marineencrustation of said component in a marine environment.
 7. A process inaccordance with claim 6, wherein the step of applying a layer of copperor substantially copper-containing material is conducted byelectroplating.
 8. A process in accordance with claim 7, wherein theelectroplating forms a layer of copper or copper-containing materialwith a minimum depth of 0.127 mm.
 9. A process in accordance with claim7, wherein the outer surface is cleaned prior to electroplating.
 10. Aprocess according to claim 9, wherein the surface cleaning includes oneor more steps of alkaline cleaning by dipping or electrolytic means,vapour degreasing and solvent cleaning.
 11. A process according to claim7, wherein the surface of the component is polished before it iscleaned.
 12. A process according to claim 7, wherein the layer of copperor copper-containing material is electroplated using an electroplatingbath which is alkaline.
 13. A process according to claim 12, wherein thecopper electroplating bath is an alkaline cyanide bath.
 14. A processaccording to claim 12, wherein the copper electroplating bath is analkaline pyrophosphate bath.
 15. A process according to claim 7, whereinthe copper layer is electroplated using an electroplating bath which isacid.
 16. A process according to claim 15, wherein the copperelectroplating bath is an acid sulphate bath.
 17. A process according toclaim 15, wherein the copper electroplating bath is an acid fluoroboratebath.
 18. A process in accordance with claim 6, wherein the component isor includes bronze.
 19. A process according to claim 18, wherein thecomponent is a bronze propeller.
 20. A process according to claim 19,wherein the bronze propeller is a newly cast bronze propeller that hasbeen first polished to propeller production standard.
 21. A processaccording to claim 20, wherein the polishing procedure includes using a60# grit size at 3500 sfm for roughing, followed by using a 180# gritsize at 5500 sfm for finishing the propeller using grease as a polishingaid.